Vanadyl in natural waters: Adsorption and hydrolysis promote oxygenation

Abstract

The aquatic chemistry of vanadium is dominated by V(IV) and V(V). Their species VO2+, VO(OH)+ and H2VO−4, HVO2−4, respectively, occur primarily in natural waters. VO2+, as a very hard Lewis acid, has a strong tendency to coordinate with oxygen donor atoms and is thus capable of both forming strong complexes with soluble organic chelates and becoming specifically adsorbed to particles, especially hydrous oxides. Vanadates (V), like phosphates, also have a tendency to form, by ligand exchange, surface complexes with hydrous oxides. The V(IV)–V(V) couple is an interesting redox sensor because the redox transition occurs at EH-values typically often encountered at the sediment water interface; organic chelate formation may extend the redox boundary to EH-values of about +0.4V (pH 7–8), but in the presence of dissolved oxygen vanadate(V) prevails. Experimental data on the adsorption of VO2+ and of V(V) (HVO2−4, VO+2) on δ-Al2O3 and TiO2 (anatase) surfaces provide evidence for strong specific adsorption. The interaction of VO2+ with oxide surfaces is interpreted in terms of inner-sphere bidentate surface complexes with the surface central metal ions of the oxide VO(OM<)2; vanadate and VO2+ form monodentate surface species. The rate of oxidation of VO2+ by oxygen is significantly enhanced by hydrolysis or adsorption to hydrous oxide surfaces. The rate law, derived earlier (Wehrli and Stumm, 1988) shows a first order dependence on the concentration of VO(OH)+ in homogeneous solution or on the concentration of the surface complex of VO(OM<)2 in heterogeneous systems. A comparison with published data on Mn(II) and Fe(II) oxidation shows that coordinated OH-groups of solid surfaces are able, like soluble hydroxo complexes, to mediate the electron transfer from the metal ions to the O2-molecule.